The present invention relates to fuel igniters for use in combustion chambers and more particularly to improved methods for providing cooling thereof.
Combustion systems for use in gas turbine engines associated with airplanes have been recently subjected to increasingly stringent exhaust emission control. Exhaust from such an engine must be relatively smokeless and include very low quantities of unburned fuel. In order to accomplish these ends, the combustion system must accomplish almost perfect burning.
On the other hand, the combustion system, like the remaining engine elements, must be designed to provide extended reliable service as well as high efficiency operation. Unfortunately, the latter characteristics are substantially antagonistic to the foregoing exhaust emission goals.
In order to thoroughly burn fuel, various means have been devised to atomize incoming fuel to extremely fine droplet sizes. An igniter is disposed in relative proximity to the atomized fuel exiting the atomizer. This disposition of the igniter results in its being projected into the area defining the primary combustion zone downstream of the atomizer. The presence of fuel-rich air around portions of the igniter result in a high rate of heat transfer into the igniter from the combustion process. Furthermore, an intense flame is frequently stabilized directly upon the igniter downstream surface. Heat transferred to the igniter in these fashions results in premature failure of the igniter, destruction through burning of associated supporting ferrules, and severe damage to the combustion chamber liner portions disposed within the wake of the flame thus stabilized.
Previous attempts at solving these problems have included the injection of cooling air into the ferrule regions surrounding a portion of the igniter, retraction of the igniter to a position more remote from the incoming atomized fuel, improving igniter construction materials, changing the igniter shape, and installing grooves in the igniter to discharge cooling air radially into the combustion zone. Each of these attempts has been partially successful but has in some way failed to achieve its intended purpose, and several have also operated to reduce the combustion ignition capability of the engine. More particularly, retraction of the igniter leads to difficulty in initiating burning due to the absence of fuel at the igniter firing tip. Changing of the igniter tip shape has aided somewhat but has not served to detach the flame stabilized thereon and has also led to weakened igniter structure. Improving the igniter construction materials is always beneficial to igniter life, but supplementary action will further improve combustion system operation. The installation of external grooves in the igniter to discharge air radially into the combustion zone serves effectively to reduce the igniter temperature but unfortunately disturbs flow fields and hampers ignition capability.